RAPID TURN-OVER OF PLASMA-MEMBRANE SPHINGOMYELIN AND CHOLESTEROL IN BABY HAMSTER-KIDNEY CELLS AFTER EXPOSURE TO SPHINGOMYELINASE

Plasma membrane sphingomyelin in baby hamster kidney (BHK-21) cells was hydrolyzed with sphingomyelinase (Staphylococcus aureus) and the effects on membrane cholesterol translocation and the properties of membrane bound adenylate cyclase and Na+/K+-ATPase were determined. Exposure of confluent BHK-21 cells to 0.1 U/ml of sphingomyelinase led to the degradation (at 37-degrees-C) of about 60% of cell sphingomyelin. No simultaneous hydrolysis of phosphatidylcholine occurred. The hydrolysis of sphingomyelin subsequently led to the translocation (within 40 min) of about 50-60% of cell [H-3]cholesterol from a cholesterol oxidase susceptible pool to an oxidase resistant compartment. The translocation of [H-3]cholesterol from the cell surface to intracellular membranes was accompanied by a paralleled increase in [H-3]cholesterol ester formation. When cells were first exposed to sphingomyelinase (to degrade sphingomyelin) and then incubated without the enzyme in serum-free media, the mass of cell sphingomyelin decreased initially (by 60%), but then began to increase and reached control levels within 3-4 h. The rapid re-synthesis of sphingomyelin was accompanied by an equally rapid normalization of cell [H-3]cholesterol distribution. The re-formation of cell sphingomyelin also led to a decreased content of cellular [H-3]cholesterol esters, indicating that unesterified [H-3]cholesterol was pulled out of the cholesterol ester cycle and transported to the cell surface. Exposure of BHK-21 cells to sphingomyelinase further led to a dramatically decreased activity of ouabain-sensitive Na+/K+-ATPase, whereas forskolin-stimulated adenylate cyclase activity was not affected. The activity of Na+/K+-ATPase returned to normal in parallel with the normalization of cell sphingomyelin mass and cholesterol distribution. We conclude that sphingomyelin has profound effects on the steady-state distribution of cell cholesterol, and that manipulations of cell sphingomyelin levels directly and reversibly affects the apparent distribution of cholesterol. Changes in the lipid composition of the plasma membrane also appears to selectively affect important metabolic reactions in that compartment.